Polyvinyl alcohol (PVA), a water-soluble synthetic resin, has been used mainly as a raw material for synthetic fibers. Recently, it is used in various fields, for example for production of film materials, emulsifying dispersants, adhesives and binder resins, because of its favorable properties. The PVA resins are generally produced by polymerization of a vinyl ester and saponification of the resulting polyvinyl ester in an organic solvent in the presence of a catalyst.
For saponification of the polyvinyl ester then, for example, a belt reactor equipped with an in-line mixer is used. FIG. 4 is a view illustrating a conventional saponification method using a belt reactor. As shown in FIG. 4, when PVA is produced by saponification of polyvinyl acetate in methanol solvent in the presence of an alkali catalyst in a conventional belt reactor, a methanol solution of polyvinyl acetate and an alkaline solution are first placed in a mixing container 101 and agitated for example in a rotor mixer (mixer) 102 for a particular time. The mixture 104 is then placed on a belt 103, where the saponification reaction is allowed to proceed under a particular temperature condition.
Also proposed conventionally were methods wherein a liquid (slurry) for saponification reaction containing a polyvinyl ester and an organic solvent and a saponification catalyst are mixed in a kneader and the saponification reaction is then allowed to proceed in a tower-type saponification reactor (see Patent Documents 1 and 2). Also proposed are a method of mixing a polyvinyl ester solution and a saponification catalyst in an in-line mixer and saponifying the polyvinyl ester in a reactor equipped with a planetary stirrer (see Patent Document 3) and an apparatus having a static mixer in a rector for saponification (see Patent Document 4).